Understanding Retaining Walls Prelude

Proper installation of small and large retaining walls is critical to assure that the wall does not fail. The following 16 pages give an overview for any retaining wall structure. These 16 pages do not encompass all aspects but are intended to give an initial and basic understanding of proper retaining wall construction.

Retaining walls are frequently thought of as a stack of blocks or wood timbers with soil dumped behind those blocks or timbers. This is a project that will fail as days, weeks and months go by. If the installer does not understand the critical aspects of constructing a retaining wall, large commercial walls or small residential landscape walls, it is probable that one of the following types of retaining wall failure will occur.

The purpose of this introduction to the Maytrx Retaining Wall Book is to give information that applies to retaining walls. The information will be given for segmental block walls although much of this information can be applied to walls built with other face materials such as wood timbers. Segmental block walls are normally built with individual stones ranging in size from about ¼ square foot for the face area of each stone to 1 square foot face area or more for each stone. Characteristically, small residential retaining walls are built with the smaller stones while large commercial walls are built with the larger stones.

Retaining Wall Knowledge

Large retaining wall projects are normally called Mechanically Stabilized Earth [MSE] retaining walls while small retaining walls are normally called Gravity retaining walls. The major difference in these two types of walls is seen in the two drawings given here.

Gravity Wall

MSE Wall

Mechanically Stabilized Earth retaining walls built today use some of the same knowledge that was used in the Great Wall of China built as much as 1600 years ago. While the earth between the parallel walls that make up the Great Wall was stabilized with water reeds laid horizontally between layers of soil, current Mechanically Stabilized Earth retaining walls are characterized by placing layers of metal straps or strong synthetic grid material in a zone of earth behind the face stones. This is done to stabilize the soil in the retaining wall as the reeds have done for 1600 years in the Great Wall of China.  Stabilizing the earth behind the face stones makes this stabilized earth zone become the principal part of the retaining wall. The face stones are a minor structural component of the stability of a Mechanically Stabilized Earth retaining wall.

Gravity retaining walls depend on the face stones to “retain” the fill soil behind the stones. The greater the set back slope [batter] of the gravity retaining wall, to a limit, the greater the ability of the stones to retain the soil placed behind the stones. Retaining walls are frequently 5 to 8 degrees off straight up. It is obvious that gravity retaining walls that are built with 0 degrees of batter, straight up, will fail with little horizontal force from the soil zone behind the face stones. It is important to take action that will minimize the horizontal force that the soil behind the stones will apply to the face stones. The proper action will be explained later in this Maytrx Book introduction. Typically, when the batter of a wall or series of tired wall exceeds 30 degrees off straight up the structure referred to as Reinforced Soil Slope [RSS] and is designed with different formulas than the formulas used to design retaining walls.

Tiered walls that exceed a 30 degree batter, RSS formulas will be used to design the overall structure

The Great Destroyer of all retaining walls is water. Referred to as hydrostatic pressure, water that is in the retained soil zone behind the retaining wall and/or the select soil zone can produce forces that will exceed or destroy the wall’s design. The principal reason is because, unlike unsaturated and compacted soil that imposes a small horizontal force on the face stones of a retaining wall, water that invades the retained soil zone or the select soil zone will put a horizontal force on the face stones that is equal to the water’s vertical force or weight. Also, too much water in the select soil zone will reduce the frictional characteristic of the select soil and then the select soil cannot provide the designed strength of the engineer’s design. Soil saturated by water will loose the internal forces that hold it in place, therefore the soil’s horizontal force on the face stones increases. Control of water in any of the soils and rock behind the face stones is perhaps the single most critical factor of both gravity and mechanically stabilized earth retaining walls. Design and construction of a swale at the surface behind the face stones of either type of retaining wall is critical if the surface soil does not slope away from the face stone toward the rear of the wall area. The swale must be designed to capture rain or other surface water and carry it away from the retaining wall. A drain tile system can be used for the same purpose.

To build a competent retaining wall of either type, gravity or MSE that will look good and serve its purpose of retaining soil for decades requires an understanding of many aspects of where the wall is to be built and how the wall will be constructed. The following information does not fulfill all knowledge necessary but will serve as an alert to owners, contractors and installers of what is important in retaining wall design. An understanding of the value of a competent design engineer for Mechanically Stabilized Earth retaining walls should be gained from this limited information.
The two types of walls will be discussed separately to eliminate confusion that may come from mixing a discussion of both types of walls in one all encompassing discussion.

Gravity Type Retaining Walls

Gravity type walls are normally for landscape applications and are usually built without geogrid or other stabilization material in the soil behind the face stones. These walls normally do not exceed 3 feet in height and if tired landscape walls are built the concepts of gravity walls may not apply. Tired landscape walls may require design as would a single wall that is of the total height of the multiple tired retaining walls.

The installer of a gravity type retaining wall needs to start by considering the location where the project is to be built. It is important that the installer study the area to determine if the soil upon which the retaining wall will rest is original virgin soil or can be compacted to carry the load of the new retaining wall. There must be no Karst topography under the retaining wall or the open cavities of the Karst topography must be stabilized. The area should be free of tree roots that may lift the face stones in the future. The installer can use paint or other marking material to mark the ground where the retaining wall will be built. Start the physical work by digging for a footing which will allow the installation of a leveling pad. The depth and width of the dig out must allow for the leveling pad’s thickness, usually about 1 inch thick for each 1 foot of wall height above the leveling pad. The leveling pad should be of fine material commonly called lime dust with some larger rocks in the mix up to 3/8 inch. This leveling pad must be compacted to a high density that will support the load of the face stones and leveled. The top of the leveling pad should be below the landscape grade in front of the wall by about 1 inch for each 1 foot of wall height above the leveling pad allowing for a toe of the retaining wall that is below the front grade. This toe prevents any sliding of the bottom layer of stones after the wall is finished.

Gravity Wall

When the leveling pad is finished the first layer of retaining wall stones can be set making certain that the stones set flat and level. If the stones do not set flat and perfect, use small handfuls of sand where needed to do micro-leveling. If more leveling is needed, remove the stones and re-level and compact the leveling pad with the lime material. When the full bottom layer of stones are set and level, a perforated drain tube should be placed behind the stones with at least one end of this tube open to an area lower than the retaining wall’s leveling pad. This tube is for infiltrate water and should never carry more than a dribble of water. Infiltrate water is water that comes from internal to the soil behind the wall and small amounts of surface water such as rain water or sprinkler system water. Rain and any other surface types of water must be directed away from the retaining wall at the surface to prevent saturation of the soil behind the wall. Saturated soil may cause a retaining wall failure where the face stones move or collapse. Around the tube and for approximately 12 inches behind the retaining wall stones, install graded gravel of 1 inch or 1½ inch size. This single size gravel will allow any water that does infiltrate into the gravel zone to percolate down to the drain tube and away from the retaining wall. Fill behind the single size gravel fill with soil that is granular in nature not a clay type soil and compact to a high density. This will minimize the soil’s tendency to hold large volumes of water. Also, fill any grove in front of the bottom layer of stones and compact to hold the toe of the wall in place. Proceed by setting additional layers of retaining wall stones, placing 12 inches of single size gravel behind the face stones and the in the cores if core type stones are used. Then place soil behind the gravel and compact building up layer by layer. Placing and compacting the fill soil layer by layer as the wall is built is critical to prevent future settlement that can cause wall failure behind the wall face stones. Compaction equipment will not full compact several feet of fill. Compact the fill material in layers of 8 inches or less to assure good soil compaction throughout the height of the fill material. Compacting the fill soil behind the retaining wall also reduces the possibility that this soil will become saturated with water that will cause the retaining wall to fail.

 

These two pictures show a retaining wall failure. The wall was not properly constructed for the application. The retaining wall is leaning forward and has separation between the individual stones. It appears that the wall stones were set with no batter. In this case it appears that water flowed into the area directly behind the wall saturating the soil behind the wall. This wall is surcharged due to the parking area near and directly behind the wall. It is likely that this application required a design engineer that would have designed the wall with geogrid to hold the surcharge of the parking lot and that a drainage system to assure that water did not collect in the soil behind the wall. A gravity retaining wall [no geogrid] may not have been adequate for this application. This small retaining wall’s failure shows that knowledge of retaining walls and proper construction is critical to all retaining wall applications. Inexperienced may lead an installer to think that gluing all stones together or the use of lip type stones will prevent a failure of a retaining wall like the one shown above. Glue and lip type stones will not prevent retaining wall failure. The hydrostatic pressure would have pushed this wall over regardless of gluing and use of lip type stones. Only proper design will prevent a retaining wall failure of the type shown in these photos.

For the top layer of the retaining wall stones it is advisable to use an adhesive manufactured specifically for retaining walls to glue the top stones to the layer below the top layer. Gluing the top layer to the layer below prevents vandalism and can avoid injury such as a single stone tipping if someone walks on the top layer.

At this point the stone part of the retaining wall structure has been completed. The soil behind the wall may be level or slope upward from the back of the face stones. Additional work is needed at the surface behind the wall to direct surface water such as rain water away from behind the retaining wall. Whether the soil is flat or slopes upward, build a swale in the soil behind the wall as shown here.

 

Design the swale such that it will flow surface water away from the wall rather than allowing the water to soak into the soil behind the retaining wall and saturating the compacted soil and rock behind the retaining wall.

A drain tile system can be designed if a swale cannot be built to carry surface water away. The drain tile may collect surface water behind the wall and carry the water away through tubes that are installed under the footing of the wall.

 

The preceding steps: 1. base soil that can carry the load of the wall with a leveling pad below grade establishing a toe to the retaining wall, 2.leveling and setting stones layer by layer with a drain tube placed in single size gravel to carry infiltration water away, 3.compacting the fill soil layer by layer behind the retaining wall and gluing the top layer to the next lower layer, 4.designing the surface soil with a swale behind the face stones to direct rain and other surface water away form behind the wall and 5. installation information specific to the retaining wall stones you use.

Following the proper installation methods will allow the gravity retaining wall you build to last for decades.

Mechanically Stabilized Earth Retaining Walls

Mechanically stabilized earth retaining walls are normally larger projects but earth stabilization with geogrid materials may be necessary in walls as short a 3 feet or less. The load to be retained determines the need of the strength of the retaining wall. A generalized view of a cross-sectional drawing of a mechanically stabilized earth retaining wall structure is given here.